There are several ways in which damage detection is performed in the prior art. According to conventional time reversal acoustics, an input signal can be reconstructed at an excitation point if an output signal recorded at another point is reversed in the time domain and emitted back to the original source point (Fink M. Time-reversed acoustics. Scientific American 1999; 281(5):91-97). This time reversibility is based on the spatial reciprocity and time-reversal invariance of linear wave equations (Draeger C, Cassereau D, Fink M. Theory of the time-reversal process in solids. Journal of the Acoustical Society of America 1997; 102(3):1289-1295. DOI: 10.1121/1.420094).
Time reversal acoustics was first introduced by modern acoustics community and applied to many fields such as lithotripsy, and ultrasonic brain surgery, active sonar and underwater communications, medical imaging, hyperthermia therapy, bioengineering, and non-destructive testing (NDT) (Edelmann G, Song H C, Kim S, Hodgkiss W S, Kuperman W A, Akal T. Underwater acoustic communication using time reversal, IEEE J. Oceanic Eng. 30, 852-864, 2005; Fink M, Montaldo G, Tanter M, Time-reversal acoustics in biomedical engineering, Annual Review of Biomedical Engineering 5: 465-497 2003; Fink M. Time-reversed acoustics. Scientific American 1999; 281(5):91-97; Beardsley B, Peterson M, Achenbach J D, A Simple Scheme for Self-Focusing of an Array, Nondestructive Evaluation 14(4) 169-179, 1995). Then, Ing and Fink adopted the time reversal process (TRP) to Lamb waves based NDT in order to compensate the dispersion of Lamb waves and to detect defects in a pulse-echo mode (Ing R K, Fink M. Time recompression of dispersive Lamb waves using a time reversal mirror—Application to flaw detection in thin plates. IEEE Ultrasonics Symposium 1996; 1:659-663. DOI: 10.1109/ULTSYM.1996.584061; Prada C, Fink M. Separation of interfering acoustic scattered signals using the invariants of the time-reversal operator. Application to Lamb waves characterization. Journal of the Acoustical Society of America 1998; 104(2):801-807. DOI: 10.1121/1.423354; Ing R K, Fink M. Self-focusing and time recompression of Lamb waves using a time reversal mirror. Journal of the Acoustical Society of America, 104(2), 801-807, 1998(a); Ing R K, Fink M. Time-Reversed Lamb Waves. IEEE transactions on ultrasonics, ferroelectrics, and frequency control 1998(b); 45(4):1032-1043. DOI: 10.1109/58.710586). The main interest of these studies was refocusing energy in the time and spatial domain by compensating the dispersive characteristics of Lamb waves.
The application of a time reversal process to Lamb wave propagations is complicated due to many factors, such as velocity and amplitude dispersion characteristics of Lamb waves and reflections from the boundaries of a structure. As a result, prior art techniques require either a human to review the results and exercise considerable judgment in order to determine the results of the test, or they require comparison to baseline data to determine if a significant change has occurred in the object being tested.
In the case of prior art solutions requiring trained human operators, the prior art solution is expensive and slow, and it has limited applicability, particularly if frequent reports are desired. For example, if NDT is to be used in applications such as testing the integrity of airplane components between every flight, then this prior art system is not practical. This solution also can lead to inconsistent results because the decision making process includes a large subjective component, and the same data can be interpreted differently by different human operators.
In the case of prior art solutions requiring the use of baseline data, there is a significant chance for false positive results as a result of factors such as changing environmental conditions. As a results, those solutions also offer significant drawbacks.
The prior art also teaches the use of guided waves. The use of guided waves has been used in Structural Health Monitoring (SHM) and Nondestructive Testing (NDT) techniques for continuous monitoring of aging aircraft, civil infrastructure and mechanical systems that have driven maintenance costs to unprecedented levels. For SHM NDT, guided waves have received a great deal of attention and have been a topic of considerable interest, because they can propagate over considerable distances with little attenuation. Conventional guided wave studies have focused on schemes where baseline signals are measured so that changes from the baseline can be detected. However, there are significant technical challenges to realizing this pattern comparison. For instance, structural defects typically take place long after the initial baseline data are collected, and other operational and environmental variations of the system can produce significant changes in the measured response, masking any potential signal changes due to structural defects.
Additionally, as discussed above, some prior art solutions require the use of a trained human operator to review the results and exercise judgment in order to determine the results of the test. As a result, these prior art solutions are expensive and slow, and they have limited applicability, particularly if frequent reports are desired. This solution also can lead to inconsistent results because the decision making process includes a large subjective component, and the same data can be interpreted differently by different human operators.
Accordingly, there is a need for improved methods, apparatuses, and systems for diagnosis testing which is both autonomous and baseline-free. In particular, there is a need for methods, apparatuses, and systems which utilize a time reversal process for diagnosis testing which is both autonomous and baseline-free. In addition, there is a need for methods, apparatuses, and systems that are less vulnerable to operational and environmental variations that might occur throughout the life span of the structures being monitored. Those and other advantages of the present invention will be described in more detail hereinbelow.